"In some developing tissues, a mass of identical cells somehow differentiates into many types. ...The mechanism behind this process appears to have evolved early on, and is highly conserved... At first this intimate mixture of fates seems puzzling, but there is a relatively easy way to achieve it: instruct each cell 'Be different from your neighbors.' This mechanism is known as lateral inhibition. An example is the nervous system. Since nerve cells form networks with long thin connections, and their ability to function depends on this geometry... when a cell develops into a nerve cell, it sends signals to nearby cells telling them not to do the same."